Active noise cancellation (ANC) is often used in ear phones and other electronic devices to cancel the noise surrounding a user. For example, users often wear headphones equipped with ANC on airplanes to drown out noise from the jet engines, as well as remove sounds from nearby passengers. Active noise cancellation typically operates by listening to external sounds, and then generating a noise cancellation signal that is 180 degrees out of phase with the actual background noise. When the ANC signal and the external sounds are combined, the external sounds are muted or at least greatly muffled.
In typical ANC applications, users will turn on the ANC function, and leave it on until they are done wearing the headset. For example, if a user is mountain biking or road biking, the user may wear ANC head phones or ear buds that allow the user to listen to music, while having outside sounds muted entirely or greatly reduced. In such an example, the user would normally typically leave the ANC feature running for the duration of the bike ride. During this ride, however, the user may miss some sounds that are important for the user to hear such as a car horn or train whistle.